Continuously operated chromatographic separation processes presently commonly employ the simulated moving bed method, which is used in a variety of different applications. The simulated moving bed method may be sequential or continuous or comprise a combination of a continuous and a sequential method.
In a continuous simulated moving bed process, all fluid streams typically flow continuously. The streams are: the supply of feed solution and eluent, the circulating of the liquid mixture, and the withdrawal of products. The flow rate for these flows may be adjusted in accordance with the separation goals (yield, purity, capacity). Normally 8 to 20 partial packed beds are combined into a loop. The eluent and feed supply and product withdrawal points are shifted cyclically in the downstream direction in the packing material bed. On account of the supply of eluent and feed solution, the withdrawal of products, and the flow through the packing material bed, a dry solids profile is formed in the packing material bed. Constituents having a lower migration rate in the packed bed are concentrated in the back slope of the separation profile, i.e. dry solids profile, while constituents having a higher migration rate are concentrated in the front slope. The points of introduction of the feed solution and eluent and the withdrawal points of the product or products are shifted cyclically at substantially the same rate at which the dry solids profile moves in the packing material bed. The eluent and feed supply and product withdrawal points are shifted cyclically by using feed and product valves located along the packing material bed, typically at the upstream and downstream end of each partial packed bed. If product fractions of very high purity are desired, short cycle times and multiple partial packed beds must be employed (the apparatus has the requisite valves and feed and withdrawal equipment).
In the sequential simulated moving bed process, some of the fluid streams do not flow continuously. The streams are: the supply of feed solution and eluent, the circulating of the liquid mixture, and the withdrawal of products (eluting phase; two to four or more products). The flow rate and the volumes of the different feeds and product fractions may be adjusted in accordance with the separation goals (yield, purity, capacity). The process commonly comprises three basic phases: feeding, elution and circulation. During the feeding phase, a feed solution, and possibly also an eluent during a simultaneous eluting phase, is introduced into predetermined partial packed beds, and simultaneously a product fraction or fractions are withdrawn. During the eluting phase, eluent is introduced into a predetermined partial packed bed or predetermined partial packed beds, and during these phases two, three or even four product fractions are withdrawn. During the circulating phase, no feed solution or eluent is supplied to the partial packed beds and no products are withdrawn.
The continuous simulated moving bed process has been disclosed in U.S. Pat. No. 2,985,589 (Broughton, et al.), for example. In accordance with this process, the mixture to be fractionated is introduced into one partial packed bed and eluent is introduced into another partial packed bed, and two product fractions are withdrawn substantially simultaneously. There are at least four partial packed beds, forming a single loop with continuous circulation, and the feed and product withdrawal points are shifted cyclically in the downstream direction in the packing material bed. A similar method is described in U.S. Pat. No. 4,412,866 (Schoenrock, et al.).
Sequential simulated moving bed processes are described in British application 2 240 053 and U.S. Pat. No. 4,332,623 (Ando, et al.); U.S. Pat. No. 4,379,751 (Yoritomi, et al.) and U.S. Pat. No. 4,970,002 (Ando et al.), for instance. A sequential simulated moving bed process applied to the recovery of betaine and sucrose from beet molasses is described in Applicants' Finnish Patent 86 416 (U.S. Pat. No. 5,127,957). In these methods, only one complete or essentially complete dry solids profile is circulated in the partial packing material loop.
Also, Applicants' copending Finnish applications 930 321 (filing date Jan. 26, 1993) and 932 108 (filing date May 19, 1993) relate to a sequential simulated moving bed method, the first applied to the fractionation of molasses and the latter to the fractionation of sulphite cooking liquor. As is described in these applications, the simulated moving bed method may include multiple loops; yet a single dry solids profile is circulated in each loop.
Finnish Patent 86 416 (U.S. Pat. No. 5,127,957) referred to above discloses a method for recovering betaine and sucrose from beet molasses employing a sequential simulated moving bed process. The chromatographic system comprises at least 3 chromatographic partial packed beds in series. In the method, betaine and sucrose are separated during the same sequence comprising a molasses feeding phase wherein the molasses feedstock is supplied to one of said partial packed beds and eluent water is supplied substantially simultaneously to another of said partial packed beds, an eluent feeding phase, and a circulating phase. These steps are repeated either once or several times during the sequence.
In the method disclosed in the above-stated Finnish application 930 321, the liquid flow is effected in a system comprising at least two partial packed beds, and the product or products are recovered during a multi-step sequence. A sequence comprises feeding, eluting and circulating phases. During the circulating phase, the liquid present in the partial packed beds with its dry solids profile is circulated in two or more loops comprising one, two or more partial packed beds. A loop may be closed or "open", in other words, when liquid is circulated in one loop, eluent can be introduced into the other loop and a product fraction can be withdrawn therefrom. During the feed and elution, the flow through the packing material beds may take place between successive loops, wherein the flows carry material from one loop to another. During the circulating phase, the loop is closed and separated from the other loops. Even in this disclosed method, only one dry solids profile is circulated in each loop.
The Applicants' Finnish application 941 866 discloses a simulated method bed method for the continuous fractionation of solutions, employing ion exchange resins of two or more different ionic forms, so that the dry solids profile formed upon passage of the solution through a chromatographic packing material having a first ionic form is passed to a chromatographic packing material having a second ionic form without the partially separated components being remixed, and/or that the concentration and pumping stages of the solution, included in the prior art methods for fractionating solutions with packed beds of two different ionic forms, can be avoided.
U.S. Pat. No. 5,198,120 (Masuda, et al.) discloses a method for fractionating a ternary or multi-component solution by a simulated moving bed method comprising a series of several columns. A circulation shut-off valve is "in between" the column series. The solution to be fractionated is supplied to the column located immediately after the shut-off valve in the downstream direction, and simultaneously one or more product fractions are withdrawn from a column located upstream. During a combined eluting and circulating phase, the solution is circulated in a loop comprising the entire column series.
European Application No. 663 224 (applicant Mitsubishi) discloses a method for fractionating a ternary or multi-component solution by a simulated moving bed method comprising a series of four columns. In this method, a loop may comprise two to four columns; yet only one dry solids profile is circulated in a loop.
In all prior art chromatographic simulated moving bed processes, only one separation profile is circulating in a loop at a given time. Accordingly, such prior art methods do not enable efficient use of the resin, resulting in lower capacity at a given product recovery, purity and resin volume.
Thus, there is a need to develop a new chromatographic simulated moving bed process which enables efficient use of the resin, resulting in higher capacity at a given product recovery, purity and resin volume.